A field effect transistor may operate in either the enhancement or the depletion mode. A device operating in the enhancement mode will exhibit no effective electrical conduction between the source and the drain regions when the gate to source voltage is equal to zero volts, assuming of course a source to drain voltage within design tolerances of the device. The minimum magnitude of gate to source voltage which will allow effective electrical current to flow between the source and the drain regions is known as the threshold voltage of the enhancement mode device. For a depletion mode device an effective electrical current may flow between the source and the drain when the gate to source voltage is equal to zero volts. The threshold voltage of the depletion mode device is the minimum magnitude of gate to source voltage which will block effective electrical conduction between the source and drain regions.
A property which is often useful in a field effect transistor is the ability to adjust the threshold voltage, or even to control whether the device operates in the enhancement or depletion mode. Typically in the prior art such control has been provided by electrically biasing the underlying substrate. While this technique is satisfactory in some circumstances, it also has significant disadvantages. Typically when a substrate bias is applied, such bias is necessarily applied to the entire substrate. Therefore a bias applied in order to control the threshold voltage of one field effect transistor will affect the threshold voltage of all field effect transistors constructed on that substrate. Furthermore if other semiconductor devices, such as bipolar transistors, are constructed on the same substrate their performance may be adversely affected by the presence of a substrate bias voltage. Alternatively other devices present on the substrate may require a particular substrate bias voltage for proper operation. Such a substrate bias may affect the operation of field effect transistors located on the substrate.
Another problem inherent in the integration of field effect transistors with bipolar devices lies in the differences between the basic processing steps used to manufacture each.